Signal Integrity

SUMMARY

Signal Integrity (SI) problems can cause unpredictable and difficult to locate failures. Design for SI from the beginning can save cost and time later. Having someone who can interpret and verify simulations rather than just produce pictures is a huge advantage.

Signal Integrity is literally the science of taking care that your signals are what you need them to be. Electronics – even Digital Electronics – relies on the size, shape and timing of the voltage waveform (signal).

When electronic components used older technologies, the speeds were relatively slow compared to those found today. For the science of SI, “speed” means the rise and fall time rather than the number of times per second that the state of the signal changes. Because a plant to manufacture components is expensive to build and run, even old components are moved to be manufactured in the new plants. Because it is the silicon technology that determines the rise and fall time, this means that in the SI sense all components are now fast.

A fast edge contains a mixture of higher and higher frequencies, and at high frequencies even a short connection on a Printed Circuit Board (PCB) behaves as a transmission line. Small changes of impedance such as when the connection passes through a via or travels on a different layer of the PCB will disturb the signal – causing losses, reflections etc. Bad Signal Integrity can cause huge levels of crosstalk between un-related signals on your board.

Why do you care? Well in the best case these effects never cause problems to the operation of your board. The next best case (although it might not feel like it at the time) is that it causes your board to fail all of the time – because then you are forced to find the problem and rectify it at the prototype stage. The worst case (and most likely) is that the problem only occurs under specific operating conditions that occur infrequently and only when your customer uses the product. Sadly Signal Integrity is normally ignored until that worst case occurs and then it is very hard to locate and solve.

Signal Integrity Measurement

It is best to consider SI from the very beginning of the design – from the layer stack up of the PCB to the geometry of the tracks on the board and the use of terminations in the circuit that you design. To acheive this the designer really needs to have a thorough understanding of the science and the experience to make the right decisions about what trade offs to make in order to achieve the aims of the design. This should be the minimum that is done.

To make a proper assessment of the SI of a design simulation is needed. To simulate a signal it is necessary to have models of the components, these are provided by the component manufacturer in an IBIS format. These models are then combined with a PCB design and simulated in a tool such as Signal Integrity Verify (SIV) to obtain the waveform that can be expected on your actual product. Many companies can run this software and produce pictures, but you really need someone who has the experience to interpret those pictures and to decide what (if any) changes are necessary to the design to make it acceptable. It is also necessary to be able to verify those simulations against real world measurements.

These techniques can be applied to solve problems with existing products, but it is more cost effective to apply them from the beginning of the design to make it extremely unlikely that an expensive problem will occur in the field.

At Dotstar Design we use the Signal Integrity Verify tool in our CADSTAR to simulate and deal with SI throughout the design process of any PCBs that we design for your project.

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